2.1, section 14.2.1 background-position states
"If at least one value is not a keyword, then the first value represents the horizontal position and the second represents the vertical position."
The order of the 2 values in the declaration for background-position matters decisively when at least one value is not a keyword. The W3C CSS validator will report a parsing error ("[values] are incompatible" due to mismatched values) when the correct order is not respected.
Firefox 1.5, Firefox 184.108.40.206, Firefox 3, Opera 9.27, Opera 9.50, Konqueror 4.1.2, Hv3 TKHTML alpha 16, Seamonkey 1.x, Seamonkey 2.x, NS 7.2, K-meleon 1.x, Galeon 2.0.4, Epiphany 2.22 all pass this test.
This bug has been reported at connect's IE beta feedback as bug 354408.
Update: This bug has been FIXED in Internet Explorer 8 Partner pre-RC1 build 6001.18343 released on December 11th 2008. Otherwise, Internet Explorer 8 Partner pre-RC1 build 6001.18343 PASSES this specific and particular testcase.